Controlling the rotation modes of hematite nanospindles using dynamic magnetic fields.

The magnetic field-induced actuation of colloidal nanoparticles has enabled tremendous recent progress towards microrobots, suitable for a variety of applications including targeted drug delivery, environmental remediation, or minimally invasive surgery. Further size reduction to the nanoscale requires enhanced control of orientation and locomotion to overcome dominating viscous properties. Here, control of the coherent precession of hematite spindles via a dynamic magnetic field is demonstrated using nanoscale particles. Time-resolved small-angle scattering and optical transmission measurements reveal a clear frequency-dependent variation of orientation and rotation of an entire ensemble of non-interacting hematite nanospindles. The different motion mechanisms by nanoscale spindles in bulk dispersion resemble modes that have been observed for much larger, micron-sized elongated particles near surfaces. The dynamic rotation modes promise hematite nanospindles as a suitable model system for field-induced locomotion in nanoscale magnetic robots.

Superparamagnetic nanoparticles with LC polymer brush shell as efficient dopants for ferronematic phases.

Liquid crystal (LC) based magnetic materials consisting of LC hosts doped with functional magnetic nanoparticles enable optical switching of the mesogens at moderate magnetic field strengths and thereby open the pathway for the design of novel smart devices. A promising route for the fabrication of stable ferronematic phases is the attachment of a covalently bound LC polymer shell onto the surface of nanoparticles. With this approach, ferronematic phases based on magnetically blocked particles and the commercial LC 4-cyano-4'-pentylbiphenyl (5CB) liquid crystal were shown to have a sufficient magnetic sensitivity, but the mechanism of the magneto-nematic coupling is unidentified. To get deeper insight into the coupling modes present in these systems, we prepared ferronematic materials based on superparamagnetic particles, which respond to external fields with internal magnetic realignment instead of mechanical rotation. This aims at clarifying whether the hard coupling of the magnetization to the particle's orientation (magnetic blocking) is a necessary component of the magnetization-nematic director coupling mechanism. We herein report the fabrication of a ferronematic phase consisting of surface-functionalized superparamagnetic Fe3O4 particles and 5CB. We characterize the phase behavior and investigate the magneto-optical properties of the new ferronematic phase and compare it to the ferronematic system containing magnetically blocked CoFe2O4 particles to get information about the origin of the magneto-nematic coupling.

Efficient ferronematic coupling with polymer-brush particles.

Switching of liquid crystal phases is of enormous technological importance and enables digital displays, thermometers and sensors. As an alternative to electric fields or temperature, magnetic fields are an interesting trigger, as they are on the one hand versatile to design, and on the other hand, they are compatible with a bouquet of applications. An interesting option to enable the magnetic switchability of nematic phases is by doping them with functional magnetic nanoparticles, but it remains a challenge to achieve well-compatibilized and stable ferronematic phases. Here, we report a new approach for the experimental realization of finely dispersed MNPs and nematic LC by creation of a surface-coupled mesogen-functionalized polymer brush, and the determination of their corresponding magneto-optical response. For this purpose, CoFe2O4 particles are equipped with a covalently attached polymeric shell carrying mesogenic groups and successfully dispersed in 4-pentyl-4'-cyanobiphenyl (5CB) to form a stable ferronematic phase at ambient concentration up to ∼1 vol%, as shown by DSC and Abbé refractometry. The magneto-optic response is detected in planar aligned LC cells. As compared to undoped 5CB, the hybrid system shows a significantly increased magnetic sensitivity, and the magneto-nematic surface anchoring is quantified by analysis of the magneto-nematic cross-correlation.

Directing the orientational alignment of anisotropic magnetic nanoparticles using dynamic magnetic fields.

The structure-directing influence of static and dynamic, i.e. rotating, magnetic fields on the orientational alignment of spindle-type hematite particles with a high aspect ratio is investigated. Structural characterization using electron microscopy and small-angle X-ray scattering confirms a nearly collinear particle arrangement with orientation of the main particle axis either parallel or perpendicular to the substrate as directed by the magnetic field geometry. The combination of large structural and magnetocrystalline anisotropies results in significantly different, strongly anisotropic magnetic properties of the assemblies revealed by directional magnetization measurements.

Magnetic and geometric anisotropy in particle-crosslinked ferrohydrogels.

Particle-crosslinked polymer composites and gels have recently been shown to possess novel or improved properties due to a covalent particle-matrix interaction. We employ spindle-like hematite particles as exclusive crosslinkers in poly(acrylamide) gels, and exploit their extraordinary magnetic properties for the realization of ferrohydrogels with a perpendicular orientation of the preferred magnetic and geometric axes of the particles. The angle-dependent magnetic properties of uniaxially oriented gels are investigated and interpreted with respect to particle-matrix interactions. The impact of the particle orientation on the resulting angle-dependent magnetic performance reveals the presence of two different contributions to the magnetization: a hysteretic component ascribed to immobilized particles, and a pseudo-superparamagnetic, non-hysteretic component due to residual particle mobility. Furthermore, a plastic reorientation of magnetic particles in the matrix when subjected to a transversal field component is observed.